The present invention relates to a water-dilutable coating composition that is particularly suitable for the coating of metals and plastics.
In the context of the painting of automobile bodies in particular it is nowadays necessary to carry out coating operations with a very wide variety of coatings. For example, automobiles that are provided with metallic paints are given a further coat of clearcoat material. On the other hand, nonmetallic paints must also be applied in the paint shops. For economic reasons, the clearcoat materials applied to the metallic paints, and the single-coat nonmetallic paints, are nowadays applied to the bodies in the same plants. This leads continually to painting defects, such as color entrainment from the colored single-coat paint into a clearcoat batch.
Nowadays, parts made of plastic are increasingly being used in automotive construction. Consequently, there is a desire on the part of the customer to paint these plastic parts as well. The coating materials at present on the market are incapable of coating metal and plastic equally without loss of quality.
Accordingly it is an object of the present invention to provide a water-dilutable coating composition comprising polyester, polyurethane, amino resins and further customary coatings additives and auxiliaries, the use of said composition preventing the painting defects which have occurred to date on the painting lines, and said composition being equally suitable, moreover, for the coating of metal parts and plastic parts.
This object is achieved by the polyurethane used being a mixture consisting of two or more, preferably two different, polyurethane compounds, (A, B) which preferably have a molecular weight of from 1000 to 30000 and preferably an acid number of from 5 to 20 mg KOH/g and are preferably preparable by reacting prepolymers containing isocyanate groups.
As binders it is preferred to use water-dilutable or water-dispersible amino resins, polyester resins and polyurethane resins which can be prepared in organic solution, and mixtures thereof.
The polyurethane resins used as binders in the base paints are known in principle. Examples of suitable polyurethane resins are those described in the literature for use in water-based coating materials, provided these polyurethane resinsxe2x80x94in modification of the preparation described in the respective literaturexe2x80x94are preparable in the form of organic solutions.
Examples of suitable polyurethane resins are the resins described in the following documents:
EP-A-355433, DE-A-3545618, DE-A 3813866 and DE-A 4005961.8
For further details of the preparation of the polyurethane resins and examples of suitable compounds, therefore, refer to these documents. Unlike the polyurethane resins described in these documents, however, the polyurethane resins are employed not as an aqueous dispersion but rather as solutions in one or more organic solvents. This means that the preparation process of the polyurethane resins used in accordance with the invention has been modified in relation to the processes described in these documents in so far as, instead of the preparation of a secondary dispersion, the polyurethane resins are dissolved in organic solvents.
It is preferred to use water-dilutable polyurethane resins which have a number-average molecular weight (determined by gel permeation chromatography using polystyrene as standard) of from 1000 to 30000, preferably of from 1500 to 20000, and an acid number of from 5 to 70 mg KOH/g, preferably of from 10 to 30 mg KOH/g, and are preparable by reaction, preferably chain extension, of prepolymers containing isocyanate groups.
The isocyanato-containing prepolymer may be prepared by reacting polyols having a hydroxyl number of from 10 to 1800, preferably of from 50 to 1200 mg KOH/g with excess polyisocyanates at temperatures of up to 150xc2x0 C., preferably from 50 to 130xc2x0 C., in organic solvents which are not able to react with isocyanates. The equivalents ratio of NCO groups to OH groups is between 2.0:1.0 and  greater than 1.0:1.0, preferably between 1.4:1 and 1.1:1.
The polyols used to prepare the prepolymer may be of low molecular mass and/or high molecular mass and they may contain anionic groups which are slow to react. In order to increase the hardness of the polyurethane, it is possible to use low molecular weight polyols. They have a molecular weight of from 60 up to about 400, and may contain aliphatic, alicyclic or aromatic groups. Amounts used in this context are up to 30% by weight of the total polyol constituents, preferably from about 2 to 20% by weight.
In order to obtain NCO prepolymer of high flexibility, a high proportion of a predominantly linear polyol having a preferred OH number of from 30 to 150 mg KOH/g should be added. Up to 97% by weight of the total polyol may consist of saturated and unsaturated polyesters and/or polyethers having a molar mass Mn of from 400 to 5000. The polyetherdiols selected should not introduce excessive amounts of ether groups, since otherwise the polymers formed undergo incipient swelling in water. Polyesterdiols are prepared by esterifying organic dicarboxylic acids or their anhydrides with organic diols, or are derived from a hydroxycarboxylic acid or a lactone. In order to prepare branched polyester polyols, it is possible to use, to a small extent, polyols or polycarboxylic acids having a higher functionality.
Typical polyfunctional isocyanates used are aliphatic, cycloaliphatic and/or aromatic polyisocyanates having at least two isocyanate groups per molecule. The isomers or isomer mixtures of organic diisocyanates are preferred. Owing to their good resistance to ultraviolet light, (cyclo)aliphatic diisocyanates give rise to products having little tendency to yellow.
The polyisocyanate component used to form the prepolymer may also contain a fraction of polyisocyanates of higher functionality, provided that this does not cause any gelling. Products which have proven suitable as triisocyanates are those formed by trimerization or oligomerization of diisocyanates or by reaction of diisocyanates with polyfunctional compounds containing OH or NH groups. The average functionality may be lowered, where appropriate, by adding monoiso-cyanates.